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The American Lung Association has released its 2013 "State of the Air" report which looks at air pollution levels across the United States, including a list of the cleanest and most-polluted U.S. cities and counties.

Read on to learn which 10 cities ranked the worst for ozone, or "smog," pollution.

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10. El Centro, Calif.

Population: 177,057

Ozone is a gas molecule that develops in the atmosphere when sunlight reactions with emissions from tailpipes and smokestacks.

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9. Washington D.C. - Baltimore - Northern Virgina

Population: 8,670,607

According to the report, more than 40 percent of the population -- or 131 million people -- live in counties that have unhealthy levels of pollution.

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8. Dallas - Fort Worth, Texas

Population: 6,853,425

Breathing in high levels of ozone can carry significant health risks for high-risk groups including infants and young children, teens, the elderly, people with lung diseases like COPD and asthma, people with heart disease, diabetics and those who work or exercise outside.

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7. Houston - Baytown - Huntsville, Texas

Population: 6,191,434

Breathing in smog causes adverse chemical reactions that can damage lung tissue, according to the American Lung Association.

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6. Sacramento - Arden Arcade - Yuba City, Calif./Nev.

Population: 2,489,230

Short-term risks from smog exposure include shortness of breath, wheezing and coughing, asthma attacks, increased risk for respiratory infections, hospitalization for those with existing lung issues, and risk for irregular heart rhythms that could one day lead to stroke or premature death.

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5. Hanford - Corcoran, Calif.

Population: 153,765

The American Lung Association urges people to skip exercising on high pollution days and never exercise near a busy freeway.

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4. Fresno - Madera, Calif.

Population: 1,095,829

And while overall air quality has improved in the United States in recent years, according to the report, many cities that ranked among the most polluted had even more days of high smog levels in 2013 compared to the previous year.

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3. Bakersfield - Delano, Calif.

Population: 851,710

The American Lung Association recommends that people ask the Environmental Protection Agency to mandate cleaner gasoline and stricter emission standards.

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2. Visalia - Porterville, Calif.

Population: 449,253

Individuals can also contribute to improving the air quality of their cities by driving less and opting for carpools, walking, biking or taking mass transit.

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1. Los Angeles - Long Beach - Riverside, Calif.

Population: 18,081,569

To see the full State of the Air report and see how your city compares to elsewhere in the country, visit the American Lung Association.